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When it grows in fast-flowing, shallow water, Porites astreoides is encrusting but in calmer water at medium depths it is a massive coral with a smooth, mounded, semi-spherical form and can grow to 60 centimetres (24 in) in diameter. At greater depths it is usually plate-like and in caves and under overhangs the plates are angled to receive the maximum amount of light. It is the only species within the genus Porites not to have a finger-like form.[2] The corallites are small and tightly-packed and give the coral a porous appearance. The polyps each have six tentacles and are generally retracted during the day. This coral is yellow, yellowish-green, pale grey or pale brown, a colour given to it by the dinoflagellatezooxanthellae, microscopic algae that live symbiotically within the coral's tissue.[3][4]

Porites astreoides is found in the tropical west Atlantic Ocean and the Caribbean Sea. Its range includes Florida, the banks off the Texas coast, the Bahamas and Bermuda. It is a common species on all parts of the reef and in lagoons.[4] It usually grows in water less than 15 metres (49 ft) deep[3] but may occasionally be found at depths of up to 50 metres (160 ft). It is sometimes free-living, growing on loose bits of coral, pebbles or mollusc shells. Large domed colonies sometimes appear to be separated into several lobes. This happens when part of the coral has died, and the living tissue in between the dead patches continues to develop, giving a clumped effect.[2]

Some colonies of Porites astreoides are female while others are hermaphroditic. Sexual reproduction occurs with male gametes being released into the sea around the time of the new moon. The sperm gets drawn into other polyps where fertilization takes place. The larvae are initially brooded but are liberated into the sea at about the time of the full moon. The planula larvae then drift with the currents for some time before settling onto the seabed and metamorphosing into new polyps. Each of these grows into a new colony by secreting a calcareous skeleton and budding. This is initially extratentacular budding, taking place at the side of the polyp, but at a later stage in the development of the colony, the budding is intratentacular, taking place within the ring of tentacles.[2]

Porites astreoides often has the sponge Mycale laevis, with prominent osculi, growing on its underside. This coral is also often associated with fan worms.[2] The polyps contain stinging cells known as nematocysts to try to ward off the stoplight parrotfish,[5] other reef fish, snails, worms and starfish that feed on it. It is also important to the coral to be in an unshaded position where its zooxanthellae can function fully. Sedimentation may limit this but under good conditions, this coral can grow at the rate of about 3.5 millimetres (0.14 in) per year.[3]